1. Field
This invention relates to ceramics generally and to stable bismuth oxide ceramics specifically.
2. State of the Art
Ceramic electrolytes, such as zirconia, have been used in automotive sensors and other applications wherein the oxygen ion transport capabilities of ceramic electrolytes have been utilized. Cubic bismuth oxide (Bi.sub.2 O.sub.3) is much more effective than zirconia as a conductor of oxygen ions. However, bismuth oxide tends to be unstable and its conductive characteristics are not retained over longer periods of time (e.g. 50 percent diminution over 100 hours) and also its strength tends to diminish. A tendency exists for the cubic phase of bismuth oxide to convert to monoclinic at temperatures below about 73.degree. centigrade (C).
Some work in the area of stabilization of bismuth oxide has occurred. Similar to stabilization of zirconia, yttria has also been used with some enhancement of stability in bismuth oxide. However, even yttria-bismuth oxide compositions do not have the long-term stability required to be an effective electrolyte for sensors and other oxygen transport devices in industry.
Bismuth oxide is known to exhibit two polymorphs; namely, cubic above 730.degree. and monoclinic below 730.degree. C. The cubic phase, which is stable between about 730.degree. C. and the melting point Bi.sub.2 O.sub.3 of 825.degree. C. is of the CaF.sub.2 type. Its chemical formula suggests that in order to crystallize in the CaF.sub.2 -type structure, there must be 25 mole % anion vacancies in the structure. The very high concentration of oxygen vacancies is believed to be the primary reason for the exceptional ionic conductivity of Bi.sub.2 O.sub.3 in the cubic form. However, pure Bi.sub.2 O.sub.3 cannot be thermally cycled through the transformation temperature since the volume change associated with the cubic to monoclinic transformation leads to disintegration of the material. It has, however, been shown that numerous rare earth and alkaline earth oxides can be added to lower the transformation temperature and thus enhance the stability range of the cubic phase. The published phase diagrams indicate that in most cases the oxide additive extends the stability range of the Bi.sub.2 O.sub.3 -oxide solid solution, which decomposes eutectoidally below about 700.degree. C. In the case of yttria (Y.sub.2 O.sub.3) as the additive, the work of Datta and Meehan shows that the cubic phase can be stabilized to temperatures at least as low as 500.degree. C. and possibly lower.
It has also been documented that oxygen ion conductivity of yttria-stabilized bismuth oxide is at least two orders of magnitude higher than that of stabilized zirconia. Thus, it would appear that yttria-stabilized Bi.sub.2 O.sub.3 would be an ideal candidate as a solid electrolyte in devices which require high ionic conductivity at moderate temperatures. Recent work has shown, however, that long-term annealing treatment at approximately 600.degree. C. leads to the decomposition of Y.sub.2 O.sub.3 -Bi.sub.2 O.sub.3 solid solutions suggesting that the phase diagram given by Datta and Meehan is incorrect. It is to be noted that Datta and Meehan, who used materials of very high purity (typically in excess of 99.99% or greater), experienced difficulty in achieving equilibrium in several of the compositions examined in their work. The significance of the purity of the starting materials will be discussed later.
Stability of the solid electrolyte is the principal requirement of any realistic device based on Bi.sub.2 O.sub.3. Thus, it is imperative that the solid electrolyte remain stable over the design life of the device. If this cannot be guaranteed, Bi.sub.2 O.sub.3 -based solid electrolytes will be of little practical value. The fact that it is possible to retain Bi.sub.2 O.sub.3 stabilized by Y.sub.2 O.sub.3 and other rare earth oxides at lower temperatures long enough to make conductivity measurements suggests that the decomposition upon annealing must somehow depend on the kinetics of mass transport. If so, the factors which suppress the kinetics of mass transport are expected to slow down the kinetics of mass transport are expected to slow down the kinetics of destabilization. Assuming that the products of the decomposition are formed by a diffusional process, it would appear that factors which suppress the pertinent diffusion coefficient would impart kinetic stability. Conversely, factors which enhance the diffusion coefficient would lead to rapid deterioration of the solid electrolyte.